DAR MANGI, Pakistan (AP) — Arifullah Khan had just administered another polio vaccine when the gunfire blasted from the nearby hills.

“It happened so suddenly. There was so much gunfire it felt like an explosion,” he said, recalling details of the attack five years ago in Pakistan’s Bajaur tribal region near the Afghan border.

A bullet shattered his thigh and he fell to the ground. His childhood friend and partner in the vaccination campaign, Ruhollah, lay bleeding on the ground in front of him.

“I couldn’t move,” Khan said. “I watched him lying right in front of me as he took his last breath.”

In Pakistan, delivering vaccines can be deadly. Militants and radical religious groups spread claims that the polio vaccine is a Western ploy to sterilize Muslim children or turn them away from religion. More than 100 health workers, vaccinators and security officials involved in polio vaccination have been killed since 2012.

The violence is an extreme example of the difficulties many poor and developing countries across Asia, Africa, the Middle East and Latin America face as they tackle the monumental task of vaccinating their populations against COVID-19.

It’s not just the problem of affording vaccines or being at the back of the line behind wealthy countries in receiving them.

Poor infrastructure often means roads are treacherous and electricity is sporadic for the refrigerators vital to preserving vaccines. Wars and insurgencies endanger vaccinators. Corruption can siphon away funds, and vaccination campaign planners must sometimes navigate through multiple armed factions.

“The most challenging areas … are conflict settings, where outbreaks of violence hinder vaccinations, and areas where misinformation is circulating, which discourages community participation,” said UNICEF’s deputy chief of global immunization, Benjamin Schreiber.

Many nations are relying on COVAX, an international system aimed at ensuring equitable access to vaccines, though it is already short on funding.

UNICEF, which runs immunization programs worldwide, is gearing up to help procure and administer COVID-19 vaccines, Schreiber told The Associated Press. It has stockpiled half a billion syringes and aims to provide 70,000 refrigerators, mostly solar powered, he said.

The agency aims to transport 850 tons of COVID-19 vaccines a month next year, double its usual annual monthly rate for other vaccines, UNICEF’s executive director Henrietta Fore said in a statement.

The situation can vary widely from country to country.

Mexico is expected to start immunizations soon. The military will handle distribution, and the government has promised free vaccines for Mexico’s nearly 130 million inhabitants by the end of 2021.

Meanwhile, Haiti, the Western Hemisphere’s poorest country, has yet to announce any vaccination plans. Health experts worry that widespread rumors could set back vaccinations — including claims that hospitals will give fatal injections to inflate COVID-19 death figures and receive more foreign aid.

The African Centers for Disease Control and Prevention is leading a continent-wide effort to vaccinate Africa’s 1.3 billion people in 54 countries. The agency is coordinating efforts to obtain doses and seeking World Bank help in funding — estimating it will take $10 billion to acquire, distribute and administer the vaccines.

The aim is to vaccinate 60% of Africa’s population within two years — some 700 million people — more than the continent has done in the past, said John Nkengasong, director of the African CDC.

“The time for action is now,” said Nkengasong. “The West cannot defeat COVID-19 alone. It must be defeated by all over the world, and that includes Africa.”

Congo underscores the obstacles the campaign faces.

The country has overcome Ebola outbreaks with vaccination campaigns. But it struggled in eastern Congo, where Allied Democratic Forces rebels stage frequent attacks and other armed groups vie for control of mineral riches.

Rough terrain and insecurity meant vaccinators had trouble getting to all areas. Some came under attack.